This allows the configuration v8_enable_shared_ro_heap and
v8_enable_pointer_compression on Linux and Android, although it still
defaults to off.

When pointer compression and read-only heap sharing are enabled, sharing
is achieved by allocating ReadOnlyPages in shared memory that are
retained in the shared ReadOnlyArtifacts object. These ReadOnlyPages are
then remapped into the address space of the Isolate ultimately using
mremap.

To simplify the creation process the ReadOnlySpace memory for the first
Isolate is created as before without any sharing. It is only when the
ReadOnlySpace memory has been finalized that the shared memory is
allocated and has its contents copied into it. The original memory is
then released (with PC this means it's just released back to the
BoundedPageAllocator) and immediately re-allocated as a shared mapping.

Because we would like to make v8_enable_shared_ro_heap default to true
at some point but can't make this conditional on the value returned by
a method in the code we are yet to compile, the code required for
sharing has been mostly changed to use ifs with
ReadOnlyHeap::IsReadOnlySpaceShared() instead of #ifdefs except where
a compile error would result due to the absence of a class members
without sharing. IsReadOnlySpaceShared() will evaluate
CanAllocateSharedPages in the platform PageAllocator (with pointer
compression and sharing enabled) once and cache that value so sharing
cannot be toggled during the lifetime of the process.

Bug: v8:10454
Change-Id: I0236d752047ecce71bd64c159430517a712bc1e2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2267300
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69174}